This invention relates to the removal of cyanide from effluents and wastes and, more particularly, relates to a method for the removal of free and combined dissolved cyanides from aqueous industrial effluents and wastes.
Due to increasingly stringent requirements for the disposal of noxious materials in waste waters and effluents from industrial processes, considerable effort has been expended, especially during the last ten years, to detoxify effluents that contain cyanides. As a result, many methods are reported in the literature that disclose the removal of free and combined cyanides from such effluents. These methods include oxidation of cyanide by treatment with chlorine, hypochlorite or ozone, treatment with an aldehyde, treatment with activated carbon or silica, removal as precipitated metal cyanides, treatment with fly ash or flue gas, and removal by irradiation, chelating, ion exchange or heating. However, in practice, most of the reported methods do not achieve the removal of cyanides and complex cyanides to levels that satisfy the requirements for safe disposal of aqueous effluents, while many methods are economically unattractive for the removal of small concentrations of cyanides from large quantities of effluent. Moreover, most methods are unsuitable to remove cyanides from slurries and solids such as flotation tailings.
We have now discovered that free cyanide as well as combined cyanides can be effectively and economically removed from effluents by treating the effluents with iron sulfides.
In the flotation concentration of mineral values in ores, cyanide ions are often used to depress certain sulfides such as, for example, iron sulfides in the concentration of lead sulfide. The depression of sulfides with cyanide ions is usually carried out at pH values of from 8 to 12.
Several methods have been disclosed that relate to the treatment of effluents with iron sulfides. According to U.S. Pat. No. 3,294,680, there is disclosed a method for reducing and removing chromate from solution by contacting the solution with a mass of hard metal sulfide, such as iron sulfide granules, at a pH of about 7.5.
According to U.S. Pat. No. 3,740,331, there is disclosed a method for the removal of heavy metal pollutant ions in a sulfide precipitation wherein sulfide ion and a heavy metal ion, which form a sulfide having a higher equilibrium sulfide ion concentration than the sulfide of the heavy metal pollutant, are added to the solution. In U.S. Pat. No. 3,901,802 there is disclosed a process for extracting heavy metal ions from solution wherein solution is contacted with an insoluble mixed salt of iron sulfide and barium sulfate containing a surplus of sulfide ions. According to U.S. Pat. No. 4,102,784, heavy metal pollutant ions are precipitated from solution by adding a slurry of large particle size ferrous sulfide precipitate to the solution in the presence of a polyelectrolyte to form a colloid-free metal ion precipitate. The foregoing methods are strictly related to the removal of heavy metal ions.
In a Technical Bulletin issued by the Permutit Co., on the Sulfex (Trade Mark) Heavy Metals Waste Treatment Process, it is stated that the process for removal of heavy metals with freshly precipitated iron sulfides at pH 8-9 is a heavy metal removal system, and that any cyanides must be oxidized in a pretreatment step. According to Japanese Kokai No. 53 101 846, which issued on Sept. 5, 1978, there is disclosed a method for the inhibition of corrosion of waste water treatment apparatus in which ferrous ions are added to waste water that contains hydrogen sulfide, ammonia and cyanide corrosion inhibitor. The water is filtered to remove iron sulfide and then treated in a stripper.